Ethylene glycol and propylene glycol are valuable materials with a multitude of commercial applications, e.g. as heat transfer media, antifreeze, and precursors to polymers, such as PET. Ethylene and propylene glycols are typically made on an industrial scale by hydrolysis of the corresponding alkylene oxides, which are the oxidation products of ethylene and propylene, produced from fossil fuels.
In recent years, increased efforts have focused on producing chemicals, including glycols, from renewable feedstocks, such as sugar-based materials. For example, US 2011/312050 describes a continuous process for the catalytic generation of polyols from cellulose, in which the cellulose is contacted with hydrogen, water and a catalyst to generate an effluent stream comprising at least one polyol.
CN 102643165 is directed to a catalytic process for reacting sugar in an aqueous solution with hydrogen in the presence of a catalyst in order to generate polyols.
As with many chemical processes, the reaction product stream in these reactions comprises a number of desired materials, diluents, by-products and other undesirable materials. In order to provide a high value process, the desirable product or products must be obtainable from the reaction product stream in high purity with a high percentage recovery of each product and with as low as possible use of energy and complex equipment.
In known processes to make glycols, the glycols are usually present at high dilution in a solvent, typically water. The water is usually removed from the glycols by distillation. Subsequent purification of the glycols is then carried out by fractional distillation. This process can have high costs both in terms of capital and operational expenditure. Further, repeated heating or maintenance at raised temperatures in the distillation steps may also lead to decomposition of the desired glycol products.
When glycols are produced by hydrogenolysis of sugars, a mixture of glycols is produced. The main glycol constituents in the reaction product stream are monoethylene glycol (MEG), monopropylene glycol (MPG) and 1,2-butanediol (1,2-BDO). The separation of these glycols by fractional distillation is complicated due to the similarity in boiling points, particularly between MEG and 1,2-BDO (respectively 197 and 196° C.). Further, the isolation of a pure MEG overheads stream by fractional distillation from a mixture comprising MEG and 1,2-BDO is made impossible by the formation of an azeotrope between MEG and 1,2-BDO.
US 2012/0184783 discloses several methods for the extraction of glycols from aqueous streams. This document discloses methods for the selective extraction of individual glycols from concentrated mixtures thereof comprising less than 50 wt % water. Further embodiments are also disclosed in which all glycols are extracted together from a more dilute aqueous stream also containing glycerol or glucose.
It would, therefore, be advantageous to provide an improved method suitable for the recovery of individual glycols from mixtures of two or more glycols, particularly MEG and 1,2-BDO.